1. Field of the Invention
This invention relates to a multi-head embroidery machine including a plurality of embroidery machines each of which comprises thread cutting means, a thread breakage sensor and a needle bar jumping mechanism.
2. Description of the Related Art
There have conventionally been provided embroidery machines for industrial use for forming the same embroidery pattern on a plurality of pieces of cloth cut into the same configuration or for forming the same embroidery patterns on a plurality of positions of a piece of cloth. For example, Japanese patent publication Nos. 4-307688 and 9-290086 disclose multi-head embroidery machines respectively. Each of the disclosed multi-head embroidery machines comprises a plurality of embroidery machines. Each embroidery machine includes a sewing bed provided with a thread cutting mechanism for simultaneously cutting a needle thread and a bobbin thread. Each embroidery machine also includes a sewing head in which a needle thread jumping mechanism for switching selectively to one of a plurality of needle bars is incorporated in a needle bar casing. Further, the sewing head of each embroidery machine is usually provided with a thread breakage sensor for detecting disconnection of the needle thread. The thread breakage sensor comprises a disc encoder rotated in synchronization with feed of the needle thread every sewing cycle and a photo-interrupter provided to interpose a circumferential edge of the disc encoder and delivering an encoder signal. In this case, disconnection of the needle thread is detected when the encoder signal is not delivered in response to a thread-breakage sensitive stitch number (5 stitches, for example).
The sewing operation is interrupted upon detection of thread breakage by the thread breakage sensor. Further, when the machine is set to an automatic step-back mode, the needle bar jumping mechanisms of all the embroidery machines are operated and a frame holder is automatically stepped back to an estimated position of thread breakage which is a predetermined number of stitches back frog a sewing interruption position.
The thread breakage is usually detected by the thread breakage sensor at a position on the cloth of the thread-breakage sensitive stitch number forward from the actual thread breakage position. Further, the machine is controlled so that the sewing processing is stopped at a sewing interruption position of a stop stitch number (5 stitches, for example) forward from a position of detection of thread breakage. Thus, in the automatic step-back mode, the position of occurrence of the thread breakage is determined to be a position back from the sewing interruption position by the number of stitches equal to addition of the stop stitch number and the thread-breakage sensitive stitch number. The thread breakage in estimated to have actually occurred at the determined position of occurrence of the thread breakage. The frame holder is controlled to be stepped back to the estimated position of occurrence of the thread breakage. Alternatively, the frame holder may be controlled to be stepped back by several stitches (three stitches, for example) so that a part of the cloth where the thread has been broken is concealed.
On the other hand, when the machine is set to a manual step-back mode, a step-back switch is manually operated after the frame holder has stopped at the sewing interruption position. The frame holder of the embroidery machine in which the thread breakage has occurred is stepped back to an actual position of occurrence of thread breakage while the step-back switch is being operated.
In each of the automatic and manual step-back modes, the needle thread is threaded through the sewing needle of the embroidery machine in which the thread breakage has occurred when the frame holder has been stepped back to the position of occurrence of thread breakage. The sewing (re-sewing) is then started. In other words, the needle bar is coupled with the needle bar jumping mechanism being stopped, and the re-sewing is carried out at a low speed from the position of occurrence of thread breakage to the sewing interruption position. At this time, the sewing machine motor is temporarily stopped. The needle thread and the bobbin thread are manually cut regarding each of the embroidery machines wherein no thread breakage occurs. The needle bar jumping mechanisms of the embroidery machines in which no thread breakage has occurred are stopped so that the needle bars are coupled. The sewing is then re-started from the position of sewing interruption wish respect to all the embroidery machines. The needle thread and the bobbin thread are fed by an amount corresponding to a maximum amount of movement of the frame holder when the frame holder is stepped back from the sewing interruption position to the position of occurrence of thread breakage. The manual thread cutting is carried out in the embroidery machines in which no thread breakage has occurred so that the fed thread can be prevented from being bitten into a thread take-up lever and unnecessary thread can be prevented from remaining on the cloth.
In the above-described conventional multi-head embroidery machine, when the thread breaks during the sewing, the frame holder is stepped back from the sewing interruption position to the position of occurrence of thread breakage. Thereafter, the needle thread is threaded through the sewing needle and re-sewing is then carried out at the low speed to the sewing interruption position. The needle thread and the bobbin thread are manually cut after the sewing operation is temporarily stopped. Thereafter, the sewing is re-started from the sewing interruption position. The needle thread and the bobbin thread need not be cut when an amount of step-back is small.
In the conventional multi-head embroidery machine, it takes a lot of time from the re-sewing to the re-start of sewing, that is, a long recovery time from the thread breakage in required. Further, there is a possibility that the needle thread and the bobbin thread are entangled with each other at the time of restarting of the sewing after the thread breakage. Further, in the embroidery machines in which no thread breakage has occurred, the needle thread and the bobbin thread are fed by the amount corresponding to the amount of movement of the frame holder due to step-back. The fed needle and bobbin threads are cut immediately before re-start of the sewing. This results in waste of the thread.